A method of making a modulation scheme multiple-valued and a method of expanding a wireless band are known as a means for increasing a transmission capacity in a wireless transmission system. As an example of the method of making a modulation scheme multiple-valued, an adaptive modulation scheme is known. A wireless transmission device using the adaptive modulation scheme automatically changes a modulation scheme and a transmission rate depending on a condition of a wireless line. As a result, the wireless transmission device using the adaptive modulation scheme is capable of improving throughput of the wireless transmission system. However, in the adaptive modulation scheme, a modulation scheme having a small multiple-valued number is generally chosen, when a line condition is degraded. Accordingly, in the wireless transmission system employing the adaptive modulation scheme, the transmission capacity decreases as the line condition is degraded. As a result, in the wireless transmission system employing the adaptive modulation scheme, data is jammed on the transmission side, a delay time increases, and data overflowed from a buffer is lost.
As a technology for expanding the wireless band, MIMO (Multiple-Input Multiple—Output) and OFDM (Orthogonal Frequency Division Multiplexing) are known. However, since a high-level wireless control technology is required in order to realize these technologies, devices thereof become expensive. Link Aggregation defined in IEEE (Institute of Electrical and Electronics Engineers) 802.3ad is known as a technology of speeding up a transmission path by putting a plurality of links together. Since the technology, however, requires a protocol processing by Link Aggregation Control Protocol (LACP), the device is still expensive.
One of solutions to the problems described above is shown in a wireless transmission system in FIG. 11. FIG. 11 is a diagram illustrating a configuration of the wireless transmission system related to the invention.
An external LAN (Local Area Network) signal 100 (MAC frame based on IEEE802.3) enters a Layer 2 switch (hereinafter referred to as “L2SW”) 50 from a user network 10. The L2SW 50 allocates the external LAN signal 100 to each MAC frame and outputs branched LAN signals 110-1 to 110-n to wireless transmission devices 60-1 to 60-n, respectively. Allocation of the frames may be conducted based on a value of a field defined in the MAC frame, like a MAC address, an IP address, and a VLAN ID (Virtual LAN Identifier).
The wireless transmission devices 60-1 to 60-n wireless-modulate inputted and branched LAN signals 110-1 to 110-n, respectively. The wireless transmission devices 60-1 to 60-n output the modulated signals to opposite wireless transmission devices 70-1 to 70-n as wireless signals 201-1 to 201-n, respectively. The wireless transmission devices 70-1 to 70-n demodulate received wireless signals 201-1 to 201-n to extract LAN signals, and output the extracted LAN signals to a L2SW 80 as branched LAN signals 510-1 to 510-n. The L2SW 80 puts the branched LAN signals 510-1 to 510-n together and outputs them to a user network 40 as an external LAN signal 500. A LAN signal is similarly transmitted from the user network 40 toward the user network 10. In the wireless transmission system shown in FIG. 11, if the number of wireless transmission devices between the L2SW 50 and the L2SW 80 is increased, a transfer capacity may be increased.
The L2SW 50 and the L2SW 80 include a Link Aggregation function without the LACP (Link Aggregation Control Protocol) as descriptions in Patent document 1. The L2SW 50 and the L2SW 80 achieve the Link Aggregation function without the LACP requiring complicated protocol processing. As described in Patent document 2 and Patent document 3, the wireless transmission devices 60-1 to 60-n, 70-1 to 70-n includes a function for putting a port of the branched LAN signal of the opposite wireless transmission device into linkdown when a linkdown state is detected at a port of the branched LAN signal.
Detecting quality degradation of the wireless signal either in the direction from the present wireless transmission device to the opposite wireless transmission device or in the reverse direction, the wireless transmission device puts the port of the branched LAN signal of the present wireless transmission device and the opposite wireless transmission device into the linkdown. As a result, the wireless transmission devices are capable of informing L2SW 50 and L2SW 80 of abnormality and stopping signal transmission.
If the L2SW in Patent document 1 is combined with the wireless transmission devices in Patent documents 2 and 3, it becomes possible to increase the transmission capacity without expanding the wireless band. Further complex LACP processing required for the Link Aggregation may be eliminated.
However, when a wireless system is configured by combining inventions described in the above Patent documents, the L2SW device becomes necessary in addition to the wireless transmission device. Therefore, total system costs increase. In addition, since the L2SW device is different from the wireless transmission device, construction and maintenance thereof is cumbersome, and it is difficult to standardize a monitoring control network.
A combination different from the combination of above Patent documents is described with reference to FIG. 12 and FIG. 13.
FIG. 12 is a diagram illustrating a configuration of a different wireless transmission system related to the invention. The wireless transmission system in FIG. 12 includes L2SW 50-1 to 50-n and L2SW 80-1 to 80-n by which L2SW 50 and 80 in FIG. 11 are replaced, and wireless transmission devices 60-1 to 60-n and 70-1 to 70-n. The wireless transmission devices 60-1 to 60-n face the wireless transmission devices 70-1 to 70-n and connect therewith, respectively.
The LAN signal 100 from the user network 10 is branched into n-signals for each MAC frame by the L2SW 50-1 which is connected with the first stage wireless transmission device 60-1. One LAN signal 110-1 is outputted to the wireless transmission device 60-1, and the other n−1 LAN signals 110-2 to 110-n are outputted to the adjacent n−1 wireless transmission devices 60-2 to 60-n. The L2SW 80-1 of the wireless transmission device 70-1 gathers the LAN signals 510-1 to 510-n which are transmitted through the wireless transmission devices 60-1 to 60-n and the wireless transmission devices 70-1 to 70-n. The L2SW 80-1 outputs the gathered LAN signals 510-1 to 510-n to the user network 40 as the external LAN signal 500.
As described, in the wireless transmission system in FIG. 12, the wireless transmission device connected with the user network connects with a plurality of wireless transmission devices adjacent thereto in a star arrangement. As a result, since the wireless transmission device connected with the user network gathers the other wireless transmission paths, extension of the transmission capacity is possible in proportion to the number of the adjacent wireless transmission devices. When a trouble occurs in the wireless transmission path, the wireless transmission device puts an internal LAN signal port connecting with the L2SW into linkdown and informs the L2SW of the trouble. The L2SW does not use the linkdown port for signal transmission based on the link aggregation function. As a result, signal transmission using only the wireless transmission device on the normal wireless transmission path becomes possible.
However, in the wireless transmission system in FIG. 12, connections from the adjacent plurality of wireless transmission devices concentrate on the wireless transmission device connecting with the user network. The wireless transmission device connecting with the user network requires interfaces, the number of which is equal to the number of the adjacent wireless transmission devices. As a result, in the wireless transmission system in FIG. 12, downsizing and price reduction of the device becomes difficult. The adjacent wireless transmission device connects only with the wireless transmission device connecting with the user network. Therefore, the adjacent wireless transmission device does not require the L2SW function. If the wireless transmission system in FIG. 12 is configured using the wireless transmission device with the L2SW function, the adjacent wireless transmission device includes the L2SW function which is not used. In the wireless transmission system in FIG. 12, therefore, price reduction of the adjacent wireless transmission device becomes difficult. If the wireless transmission system in FIG. 12 is configured by eliminating the L2SW function from the adjacent wireless transmission device, the wireless transmission device with the L2SW function and the wireless transmission device without the L2SW function exist in the wireless transmission system. In this case, construction and maintenance of the wireless transmission device becomes cumbersome.
FIG. 13 is a diagram illustrating another wireless transmission system related to the invention. The internal configuration of the wireless transmission system in FIG. 13 is similar to that of the wireless transmission system in FIG. 12. The wireless transmission system in FIG. 13 differs in connection between devices from the wireless transmission system in FIG. 12.
That is, in the wireless transmission system in FIG. 13, the wireless transmission device 60-1 connecting with the user network 10 connects with the adjacent wireless transmission device 60-2, and the wireless transmission device 70-1 connects with the adjacent wireless transmission device 70-2. In the wireless transmission system in FIG. 13, the wireless transmission device 60-2 connects with the adjacent wireless transmission device 60-3, and the wireless transmission device 70-2 connects with the adjacent wireless transmission device 70-3. In the wireless transmission system in FIG. 13, all the wireless transmission devices are similarly connected in cascade.
In the wireless transmission system in FIG. 13, when the adjacent wireless transmission devices are connected in cascade, the plurality of wireless transmission paths are put together and extension of the transmission capacity becomes possible depending on the number of the wireless transmission devices. In addition, the wireless transmission system in FIG. 13 detects abnormality in the wireless transmission path or a wireless transmission circuit, puts the internal LAN signal port which is connection with the L2SW into linkdown, and informs the L2SW of the abnormality. The L2SW does not use the linkdown port for signal transmission based on the link aggregation function. Therefore, signal transmission only using the wireless transmission device on the normal wireless transmission path may continue.
In the wireless transmission system in FIG. 13, since all the wireless transmission devices include the link aggregation function-enabled L2SW, price reduction of the device is difficult. When a simple and low price L2SW without the link aggregation function is applied to the adjacent wireless transmission device, the wireless transmission device connecting with the user network and the adjacent wireless transmission device which are different from each other are used, and therefore construction and maintenance thereof becomes cumbersome.